1. Field of the Invention
The invention pertains to a drive sprocket wheel for a track-type undercarriage, especially for lattice mast cranes, which sprocket wheel has track teeth around its circumference and works together with an endless track. The endless track has individual track links, each with its own base plate, where each track link has two spaced-apart guide webs which are symmetric to the center of the link; a cog extending downward between the links meshes with the track teeth of the sprocket wheel.
2. Description of the Related Art
A sprocket wheel of the general type in question is known from GB 1,602,143. This works together with an endless track and is provided with track teeth of the involute type. The endless track consists of multiple track links, each with its own base plate and with two guide webs asymmetric to the center, spaced a certain distance apart, and having engaging surfaces which ride on circumferential flanges on the sprocket wheel. A cog connects the two webs from a position below the engaging surfaces and meshes with the teeth of the sprocket wheel. The transverse pitch or pitch angle, which is obtained by dividing 360 degrees by the number of track teeth, is constant over the entire circumference. The disadvantage of the known solution is that, when a crane is traveling downhill, it must be braked. As a result of the clearance present in all segments, a relative displacement occurs between the sprocket wheel and the endless track, so that the head area of the track tooth runs up against the flank of the cog. In the least favorable case, the track tooth shears off the flank of the cog, which leads to a further deterioration in the meshing conditions, as can be recognized from the loud noise produced. The problems just described also occur when the change is made from forward to reverse travel.
A camshaft drive for passenger car engines in the form of a sprocket wheel chain drive is known from DE 43 31 482, which, to suppress howling noises caused by resonance vibrations, provides sprocket wheels with varying pitch and a chain with links of different lengths corresponding to the varying pitch of the sprocket wheels. The problems of improving the meshing between the sprocket wheel and the chain is not addressed in this document.
The task of the invention is to improve the sprocket wheel of the general type in question in such a way that the disadvantages described above are avoided.
According to the invention, the toothed rim area of the sprocket wheel has at least two circumferential sections, which, while the tooth width remains constant, are provided with a transverse pitch which varies from segment to segment, one xe2x80x9csegmentxe2x80x9d being defined as the intermediate space between the centers of two immediately adjacent teeth. The rule applies here that the sum of all deviating transverse pitches divided by the number of teeth results in a value which corresponds to the constant transverse pitch. Another characteristic is that, relative to the rotational direction of the sprocket wheel, the transverse pitch decreases from a maximum value to a minimum value or increases from a minimum value to a maximum value in each circumferential section, and also that the greatest difference in the transverse pitch occurs at the transition from one circumferential section to the next. The smallest occurring minimum value of the transverse pitch is selected so that, relative to the cog of the endless track engaging in the spacewidth, a clearance remains, which, under consideration of the production tolerances for the endless track and the sprocket wheel and the wear of the endless track, makes it possible for the cog to engage in the spacewidth. With respect to production tolerances, the least favorable pairing (i.e., cog at the upper tolerance limit and spacewidth at the lowest tolerance limit) is to be taken into account, and, with respect to wear, essentially the stretching of the endless track is to be considered. The endless track used with the sprocket wheel according to the invention has links of uniform length, as is conventional.
The proposed design offers the advantage that the previously known difficulties not longer occur, especially those associated with traveling downhill, because the endless track is held by the segment of the sprocket wheel with the narrowest pitch, and thus it is impossible for relative motion to occur between the sprocket wheel and the endless track.
The toothed rim area preferably has at least two similar (rotationally symmetric to the rotational axis of the sprocket wheel) circumferential sections, which, whereas the tooth width remains constant, are provided with a transverse pitch which changes from segment to segment, where the sum of all deviating transverse pitches in the circumferential section in question divided by the number of track teeth of this section results in a value which corresponds to the constant transverse pitch.
In cases where the transverse pitch decreases in the rotational direction of the sprocket wheel, the total clearance between the cog and the spacewidth decreases from a maximum value to a minimum value in the segment formed by a trailing and a leading tooth. The reverse is true in the case of an increasing transverse pitch.
The total clearance preferably consists of a clearance component A formed between the leading flank of the trailing tooth and the trailing flank of the cog and a clearance component B formed between the trailing flank of the leading tooth and the leading flank of the cog, where the sum of clearance component A and clearance component B is different in each segment.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.